Three lines of experimentation are proposed: 1. A study is being conducted of the structure of the chromatin containing the oocyte-type 5S RNA genes in Xenopus laevis. Experiments to date have shown that these genes are in a typical nucleosome structure in somatic tissues, where they are not being expressed. Investigation will now be made of "active" 5S DNA from oocytes, with particular emphasis on the presence of nucleosomes and the possibility of phasing of the histones with respect to the DNA sequence. In addition, a search is being initiated for proteins which bind specifically to 5S DNA, in the hope of finding transcriptional regulators. 2. A new satellite DNA has recently been isolated from X. laevis, and its characterization is continuing. From nucleotide sequence analysis, an evaluation of its coding capacity will be made. Nucleic acid hybridization experiments will be conducted to determine its abundance in total genomic DNA, the existence of possible transcripts in cellular RNA, and its chromosomal location. 3. The recombination behavior of X. laevis 5S DNA is being investigated using specially constructed bacteriophage lambda vectors. Unequal crossovers in the internally repeated sequences of the spacers, if they occur, would have implications for the evolution of highly reiterated gene families.